For me it's being able to support yourself and your family in a fairly comfortable western middle class quality of life. The FU part is that, if you're making this independently and reliably, you don't need to care about the TechCrunch/etc bullshit the OP mentioned.
For another person it might mean living in a West Village townhouse and sending your kids to private schools. For yet another it's owning a private jet and the 49ers. In these cases you probably need more than $10k/mo.
$10K/month would give me freedom to spend 4+ hours with my son/day and not give a crap about how to get php+jquery+mysql to behave sensibly today. What do I care about FU money? If x$ gives you freedom, by definition it is FU money.
$10k/month is most certainly enough money for me to quit my 9-5 and live a comfortable life doing what I want to instead of what I have to. That's FU money as far as I'm concerned.
